1. Field of the Invention
The present invention relates to power conversion systems, and more particularly to a method and apparatus for a modular and scalable power conversion system for an aircraft.
2. Description of the Related Art
Electric systems used in complex environments such as aerospace systems, more electric aircraft systems, industrial environments, vehicles, etc., include a large number of electric systems and modules. During operation of such complex environments, various electric systems and modules may need to be connected to electric power sources, disconnected from electric power sources, maintained in a powered-up state, etc., at various times. Moreover, various electric systems and modules in a complex environment may require different amounts and type of electrical power. For example, some electric systems and modules may require DC power while others may require AC power. Some electric systems and modules may require 28 Vdc, others 230 Vac, yet others 115 Vac at 400 Hz. The power levels required by various parts of a complex environment may also depend on the operational stage of the environment. For example, different levels of power may be needed during a start-up and during a continuous operation of a complex environment, such as an aircraft.
Aircraft are currently being designed to use less non-electric power (such as hydraulic and pneumatic power) and more electrical power. Aircraft system architectures that rely solely, or to a great extent, on electrical power, are also referred to as More Electric Aircraft (MEA) system architectures. Typically, MEA system architectures use starter-generators to start the aircraft main engines, as well as supply electrical power to various system loads that may utilize electrical power at various frequencies and voltages. Hence, many MEA system architectures, and/or starter-generators currently used to power MEA system architectures, typically include relatively complex power electronics circuits with large weight. In these heavy power electronics circuits, motor controllers are used for main engine start and after the start, to supply the motors in the Environmental Control System (ECS) or other motor loads in the aircraft systems, such as hydraulic system loads.
One such power system architecture for aircraft is described in patent application US 2004/0129835 A1, by W. Atkey et al. In this patent application, an electric power distribution system includes AC generators. High voltage AC power can be converted to high voltage DC power by one or more AC-to-DC conversion devices, such as auto transformer rectifier units (ATRUs), that receive AC power from AC busses. Using the ATRUs, the power distribution system provides high voltage AC and DC power to support conventional 115V and 28 Vdc bus architectures. An output from an ATRU is alternatively connected to an AC generator during start, and to a load such as an air compressor system, during normal operation.
However, typical/conventional power conversion systems place design constraints on the generating and conversion equipment such as the motor controllers, since the design of the generating and conversion equipment is heavily dependent on the larger power typically required to achieve the main engine start. The output current required for main engine start is 2 to 5 times larger than the current required to drive motors in the ECS or in other systems. This discrepancy in power requirements leads to designs with large output ratings, and imposes weight, volume and cost penalties on existing aircraft systems, resulting in sub-optimal approaches to the design of architectures used for MEA. Moreover, in typical/conventional power generation and conversion systems, the availability of the start system is negatively affected, because a failure of one of the motor controllers used for start removes at once the start capability for the starter generator associated with the failed controller.
Disclosed embodiments of this application address these and other issues by utilizing a modular and scalable power conversion system consisting of power conversion modules, which are designed and optimized for continuous operation when they supply motors used in aircraft systems, or aircraft busses with fixed frequency. During main engine start, a number of power conversion modules are operated in parallel and used to supply start power to a starter generator. The power conversion modules may be controlled for connection to any starter generator or motor in the electric system, hence allowing for power conversion modules to be designed for much lower ratings, to realize weight, volume and cost savings. In the power conversion system described in the current application, the availability of the start system is increased over previous systems, because a failure of one of the power conversion modules used in parallel during start will remove only partially the start capability, as the other connected power conversion modules are able to supply start power.